fix(sdd): batch small same-shape tasks into one dispatch

Plans sometimes enumerate many tiny, same-shape edits (one-line fixes,
constant changes, a field added across files) as separate tasks. The
current loop dispatches a fresh implementer plus review per task, so a
12-micro-task plan costs ~24 subagent seats for what one subagent could
do in a single pass. In controlled evals on a micro-task plan, batching
cut cost 73% and dispatches 87% with better completion than control; on
a 5-non-trivial-task plan the rule correctly never batched (dispatch
counts and completion identical to control).

Claude-Session: https://claude.ai/code/session_0185AJr98gHx5EmwqNeft4Sy

skills/subagent-driven-development/SKILL.md

@@ -193,6 +193,14 @@ that implementer. Single-file mechanical fixes also take the cheapest tier.
 
 ## The Task Loop
 
+**Batch small same-shape work.** When the plan lists several tasks that are
+each a small, independent edit of the same kind — the same one-line fix,
+constant change, or field addition repeated across files — do not dispatch
+one subagent per task. Compose ONE dispatch brief listing every file and
+its change, send the whole batch to a single subagent, and review its diff
+as one unit. Reserve one-dispatch-per-task for work that needs its own
+judgment, its own tests, or its own review surface.
+
 Everything you paste into a dispatch prompt — and everything a subagent
 prints back — stays resident in your context for the rest of the session
 and is re-read on every later turn. Hand artifacts over as files.